This invention relates to a process for irreversibly denaturing and, at the same time, deodorizing biomasses containing microorganisms and the secondary products thereof, particularly biological clarified sludges, by reaction with compounds containing isocyanate groups. According to the present invention, the thus-obtained substantially odorless, denatured polyaddition products are used as reactive fillers or molding compositions in the production of plastics or as long-term fertilizers in agriculture.
In biological purification plants, organochemical effluent impurities are degraded, i.e. biologically eliminated, by means of microorganisms. Under the conditions applied, the microorganisms multiply to a particularly marked extent. The quantity of the biomass consists mainly of bacteria in the so-called "activated sludge basin" of the purification plant increasing daily by from about 3 to 4%, by weight, so that, although some of the microorganisms die, the quantity of bacteria would double in from 3 to 4 weeks. Accordingly, some of the biomass has to be continuously removed from the activated sludge basin in the form of so-called "surplus activated sludge" in order to maintain the optimal conditions for microbial effluent purification. For this reason, biomasses accumulate worldwide in extremely large and ever-increasing quantities in the fully biological purification of industrial and communal effluents. In the Federal Republic of Germany alone, about 2 million metric tons (expressed as dry weight) per year of these protein-containing biomasses are at present either being dumped or burned. Even today, the necessary removal of the water from the activated sludges is still a problem because, under the sedimentation conditions normally applied in the purification plants, the activated sludge to be removed contains only about 1%, by weight, of microbial dry mass. In conventional centrifuges, the solids content of the sludge may only be concentrated to from 7 to 9%, by weight. Even where polyelectrolytes are added and centrifugal decanters used, it may only be increased to from 12 to 15% by weight.
Even in these low concentrations, the activated sludges have a pronounced gel structure and a relatively high viscosity on account of the pronounced chemical and physical binding of the water to the microorganisms. For this reason, normal filtration is impossible without certain treatment. Filtration is also complicated by the fact that the bacteria cells attract one another and form common, slimy shells, resulting in the formation of tacky flakes. In practice, therefore, inorganic primary sludges are added to the surplus activated sludges in from substantially the same to twice the quantity in order to facilitate removal of the water on an industrial scale by means of filter presses. In this way, a filter cake having a very high content of inorganic constitutents and a water content of about 50%, by weight, based on the mass as a whole, is obtained. On the other hand, burning may only be carried out using surplus activated sludge powders having a very high content of organic mass. This is done either under substantially anhydrous conditions with the disadvantage that the drying process requires far more energy than may be obtained as heat equivalent during burning, or aqueous activated sludge is burned with an addition of, for example, heavy oil as energy carrier in a quantity sufficient to evaporate the quantities of water entrained.
Another problem is that, as soon as it is isolated from the settling basin, the excess bacterial sludge immediately begins to rot and gives off an unbearable odor. Even anhydrous activated sludge powder dried at 110.degree. C. has a very unpleasant odor and continues to rot on becoming moist. The presence of pathogenic germs cannot be ruled out.
For these reasons, the composting of the treated sludge or its direct use as a fertilizer in agriculture is possible only to a limited extent. The elimination and utilization of treated sludges involve considerable ecological problems which have not been solved in a satisfactory manner. Known processes for working-up biomasses of microorganisms and the disadvantages and inadequacies thereof are discussed in detail in U.S. patent application Ser. No. 84,002, now abandoned.
According to the estimates of the Federal Ministry of the Interior of the Federal Republic of Germany (1975 Waste Economy Program of the Federal Government; Environmental Letter 13, 1976), the annual accumulation of treated sludge will have increased by 1985 to about 50 million cubic meters from communal plants plus another 30 million cubic meters from industrial plants, which for a water content of 95% represents approximately 4 million metric tons of dry purified sludge per year. Accordingly, it is urgently necessary both for ecological reasons and also for economic reasons to find improved processes for working up surplus activated sludges with elimination of harmful impurities. It is also necessary to enable the purified sludges consisting mainly of high-quality proteins, nucleic acids enzymes and other valuable organic compounds to be utilized without endangering the environment by recycling on an industrial scale.
It has now surprisingly been found that various biomasses based on microorganisms or metabolism and/or decomposition products thereof, including in particular the above-described purified sludges from biological purification plants, may be worked-up in a simple and considerably improved manner. This is accomplished by reacting the biomasses with compounds containing isocyanate groups. The reaction may take place optionally in the presence of organic solvents, carbonyl compounds, compounds capable of aminoplast or phenoplast formation and/or other additives, optionally at elevated temperature and/or elevated pressure. In the context of the present invention "working-up" is to be understood to mean that the biomasses are concentrated, irreversibly deodorized and, in this way, made available for utilization in the plastics-processing industry and in agriculture. The biomass polyaddition products obtained in accordance with the present invention are sterile, completely odorless in most cases and contain the biomass used in chemically bound and completely denatured form. The products are not tacky in aqueous phase, may be filtered without difficulty and dried in energy-saving manner. They are completely stable in storage and free from pathogenic organisms. The total enzyme deactivation and complete cell death of the biomasses completely suppress decomposition and putrefaction processes, fermentation and unpleasant odor formation of enzymatically or microbiologically degradable cell ingredients. Accordingly, the process products may be indefinitely stored both in dry and also in moist form without giving off unpleasant odors and without undergoing further enzymatic degradation.
It is known that isocyanates may be reacted with other starting materials of the type commonly encountered in polyurethane chemistry in the presence of biologically active substances to form high molecular weight compounds. In contrast to the present invention, whose object is to denature biomasses based on microorganisms with complete destruction of living cells and active enzymes present therein, the known processes seek to fix selected biologically active compounds in polyurethanes with full retention of the bioactivity.
Thus, in German Offenlegungsschriften Nos. 2,612,138 and 2,625,544, for example, describe the fixing of enzymes, antigens, antibodies or antibiotics by means of prepolymers containing isocyanate groups. In this case, the polyaddition reaction has to be carried out very carefully to avoid destruction of the bioactive substances. The thus obtained products are used as biospecific catalysts, antigens or antibodies. Various biologically fully active or even activated substances fixed to a polyurethane matrix may be similarly produced in accordance with German Offenlegungsschriften Nos. 2,319,706 and 2,625,471 and U.S. Pat. Nos. 3,574,062; 3,705,084; 3,791,927; 3,672,955; 3,929,575; and 3,905,920. As mentioned above, the process according to the present invention differs from these known processes not only in regard to the starting materials used (microbial biomasses of extremely heterogeneous composition which still contain virtually all the cell constituents and, in general, even have a largely undamaged cell structure and also contain living cells are used instead of isolated biochemically active individual compounds), but also in regard to the reaction conditions so that the biomasses treated in accordance with the present invention are completely changed physically, chemically and biologically in relation to the starting material.